Method and apparatus for electrolytically plating copper

ABSTRACT

A method for electrolytically plating copper onto a base metal, such as steel (18), utilizing an insoluble anode (16) includes the steps of: providing a pyrophosphate plating solution (14); adding a copper source, copper hydroxide, to the plating solution (14); and passing an electric current through the plating solution between the insoluble anode (16) and the base metal (18) to be plated. The apparatus (10) includes a plating tray (12), a pyrophosphate plating solution (14) including a soluble source of copper, an insoluble anode (16) and a power source (20). A copper plated product (P) is also disclosed and claimed.

TECHNICAL FIELD

The present invention relates generally to the plating field and, moreparticularly, to a unique method and apparatus for plating copper onto abase metal such as steel utilizing an insoluble electrode and copperions from a plating solution.

BACKGROUND OF THE INVENTION

Steel, plated with copper, is utilized for a wide variety ofapplications. For example, copper plated steel wire is utilized for tirecord in steel radial tires, in high pressure hoses and belts and hasother related applications.

Prior art electrolytic copper plating methods have utilized platingsolutions commonly formed from copper pyrophosphate and copper sulfate.The copper pyrophosphate solution is understood to be pH neutral and isoften preferred over the pH acidic solution of copper sulfate. No matterwhich solution is selected, however, prior art apparatus and methodshave utilized soluble copper anodes.

For example, oxygen-free copper metal is usually utilized in a solubleanode in a copper pyrophosphate plating solution. The anode is placed ona positive charged electrode basket made of titanium or stainless steel.During plating, the anode changes shape. More specifically, copperdissolves from the anode to replace copper consumed from the solution toplate the steel. This change in shape, disadvantageously, results inrelatively large variations in the current density at the steel beingplated (functional cathode). This leads to uneven plating on the steel.Accordingly, plating quality is adversely effected.

An additional drawback to the utilization of soluble anodes is the needto periodically provide replacement as the copper of the anodes becomesexhausted. This is an inconvenient, relatively time consuming andusually unpleasant task. In many plating operations, it also may requiresome downtime which adversely effects productivity.

In others, and particularly continuous plating operations, "sparking"may occur during anode replacement. Sparking results when the anodesimultaneously contacts both the electrode basket (positive charge) andthe steel being plated (negative charge). Sparking creates a surfacedefect which is detrimental to the finish of the plated product.

SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providean improved method and apparatus of copper plating specificallyovercoming the above-described limitations and disadvantages of theprior art.

A more specific object of the present invention is to provide a novelcopper plating method and apparatus providing significantly enhancedcharacteristics including more uniform plating over the entire surfacebeing plated.

Still another object of the invention is to provide a method andapparatus for plating copper on steel that substantially eliminates thedowntime and lost productivity associated with the replacement ofsoluble anodes in non-continuous plating operations.

Yet another object of the invention is to provide a plating method andapparatus utizling insoluble anodes so as to provide for the eliminationof the sparking problem commonly occurring when soluble anodes asutilized in the prior art are replaced during continuous platingoperations. As a result, the surface defects associated with sparkingare virtually eliminated and, accordingly, a product of improved overallquality is produced.

A still further object of the invention is to provide a method andapparatus for copper plating steel wherein an insoluble anode isutilized and a separate copper source is added to the plating solutionas required to provide a high quality, consistent and uniform platingoperation. Further, this is achieved while eliminating the need toperiodically replace the anodes thereby eliminating this unpleasanttask.

Yet another object of the invention is to provide a copper plated steelproduct of improved quality produced in accordance with the presentmethod.

Additional objects, advantages and other novel features of the inventionwill be set forth in part in the description that follows and in partwill become apparent to those skilled in the art upon examination of thefollowing or may be learned with the practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention as described herein, a novel methodfor electrolytically plating copper onto any base metal upon whichcopper may be plated is provided. The method utilizes a platingapparatus including a power source, a plating tray and an insolubleanode.

More particularly, the method includes the providing of a pyrophosphateplating solution in the plating tray. Preferably, the pyrophosphateplating solution is formed from water, 200-400 grams per liter oftetrapotassium pyrophosphate (K₄ P₂ O₇) and 40-60 grams per liter ofcopper pyrophosphate (Cu₂ P₂ O₇). Additionally, the plating solutionincludes between 0-3 grams per liter of polyphosphoric acid. Theaddition of this acid allows an adjustment to be made to the initial pHof the plating solution. Specifically, the initial pH is brought tobetween 7 and 10 so as to make a selected copper source soluble therein.

Thus, the method also includes the step of adding a copper source to theplating solution. Preferably, the copper source is copper hydroxide(Cu(OH)₂). Advantageously, the copper hydroxide is fully soluble in thepyrophosphate plating solution and effectively serves to provide copperions for plating and hydroxide ions for reacting with hydrogen ionsproduced during the plating process. Accordingly, the copper hydroxidefunctions to maintain the pH level of the plating solution relativelyconstant during the plating process while producing water as theneutralization reaction product. Advantageously, the water byproduct ofthe plating process is easily handled in an environmentally safe manner.Further, it should be appreciated that there is no buildup ofprecipitates or hazardous neutralization products in the platingsolution even after continuous operation over an extended period oftime.

Plating is completed by passing an electric current through the platingsolution between the insoluble anode and the base metal, e.g. steel, tobe plated. This current effectively drives the plating process. Ascopper ions are plated from the solution onto the steel, they arereplaced in the plating solution by the addition of more copperhydroxide. Specifically, during the plating process, a concentration ofbetween 21.5-33.8 grams per liter of copper hydroxide is maintained inthe plating solution. Advantageously, this level of copper hydroxidealso serves to maintain the plating solution at a pH of between 7-10; arange in which copper hydroxide is soluble in the pyrophosphate platingsolution.

For the most efficient and effective plating, a current density ofsubstantially 8-10 and more preferably 9 amps/dm² is maintained bypassing an electric current of substantially 50 amperes at substantially8 volts. Additionally, the plating solution is maintained at atemperature of between 45° and 55° and more preferably substantially 50°C. Further, the concentration of copper in the plating solution ismaintained between 14 and 22 grams per liter and more preferably atsubstantially 20 grams per liter. Advantageously, as the plating processis completed with an insoluble anode that maintains a constant shape andmass, the current density does not fluctuate, and accordingly, a veryuniform and high quality plating of copper on steel is achieved.Further, as there is no need to replace anodes and the process may becompleted in a continuous manner, the prior art problem of sparking iseffectively eliminated and accordingly, surface defects associatedtherewith are also eliminated. Excellent productivity is also assured.

In accordance with a further aspect of the present invention, anapparatus for copper plating a base metal such as steel is alsoprovided. The apparatus includes a plating tray formed from anon-corrosive material such as stainless steel. A pyrophosphate platingsolution of the type described above including a source of copper forplating is held in the plating tray. Additionally, an insoluble anode oranodes are installed in the plating tray and covered with the platingsolution. Preferably, the anodes are made of titanium (Ti) coated witheither iridium dioxide (IrO₂) or a combination film of iridium dioxideand platinum (Pt).

Still further, the apparatus includes a power source, such as arectifier, for passing electric current through the plating solutionbetween the anode and the steel so as to plate copper from the solutiononto the steel in a uniform manner.

In a more preferred arrangement, the apparatus also includes a separatedissolving tank particularly adapted for dissolving copper hydroxideinto a pyrophosphate plating solution. A conduit provides fluidcommunication between the dissolving tank and the plating tray. A pumpserves to supply plating solution including copper hydroxide from thedissolving tank to the plating tray thereby replenishing the coppersupply for plating as needed.

Additionally, the apparatus includes pH monitors for monitoring the pHof the plating solution in the plating tray and the pH of the platingsolution in the dissolving tank. As the pH of the plating solutionapproaches the lower end of the pH range 7-10 in the plating tray, it isnecessary to pump plating solution including relatively higher levels ofcopper hydroxide and, accordingly, an associated higher pH from thedissolving tank to the plating tray. In this way, the desired pH rangeand levels of copper hydroxide are maintained throughout the platingprocess.

Finally, in accordance with yet another aspect of the present invention,a copper plated steel product produced in accordance with the methoddescribed above is provided. The product is characterized by a uniform,high quality copper plate finish.

Still other objects of the present invention will become apparent tothose skilled in this art from the following description wherein thereis shown and described a preferred embodiment of this invention, simplyby way of illustration of one of the modes best suited to carry out theinvention. As it will be realized, the invention is capable of otherdifferent embodiments, and its several details are capable ofmodification in various, obvious aspects all without departing from theinvention. Accordingly, the drawings and descriptions will be regardedas illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing incorporated in and forming a part of thespecification, illustrates several aspects of the present invention andtogether with the description serves to explain the principles of theinvention. In the drawing:

FIG. 1 is a schematical representation of the apparatus of the presentinvention.

Reference will now be made in detail to the present preferred embodimentof the invention, an example of which is illustrated in the accompanyingdrawing.

DETAILED DESCRIPTION OF THE INVENTION

Reference is now made to drawing FIG. 1 showing the apparatus 10 of thepresent invention for plating copper onto a base metal upon which coppermay be plated. The apparatus 10 and method are described with referenceto plating steel wire 18. It should be recognized, however, that othermetals may be plated and that steel is only being referenced forpurposes of illustration.

As shown, the apparatus 10 includes a plating tray 12 that is filledwith a plating solution 14, such as a pyrophosphate solution. Thepyrophosphate plating solution 14 preferably includes: water; between200-400 grams per liter of tetrapotassium pyrophosphate (K₄ P₂ O₇);between 40-60 grams per liter of copper pyrophosphate (Cu₂ P₂ O₇); andbetween 0-3 grams per liter of polyphosphoric acid. More preferably, theplating solution 14 includes substantially: 300 grams per litertetrapotassium pyrophosphate; 56 grams per liter copper pyrophosphate;and 1 grams per liter polyphosphoric acid. For best results, thesolution is maintained during plating at 45°-55° C. and more preferably,substantially at 55° C.

One or more anodes 16 are provided near the bottom of the plating tray12 in the plating solution 14. Each anode 16 is of the insoluble type,preferably formed from titanium (Ti) coated by iridium dioxide (IrO₂),or a combination film of iridium dioxide and platinum (Pt).

The steel 18 to be plated is passed through the plating tray 12 belowthe surface of the plating solution 14 in a manner known in the art asshown. For example, the steel 18 may be in the form of wire having adiameter of 1.0-2.0 mm. The steel 18 is maintained in the platingsolution 14 during processing for a total time of approximately 40-50and more preferably 45 seconds. When a continuous steel wire 18 isplated, the wire may be moved through the solution 14 in the platingtray 12 at a speed of at least up to 2.6 m/min. depending upon thelength of the plating tray, the desired depth of plating and the currentdensity being utilized.

The power required to complete the plating operation is provided bymeans of a power source 20, such as a rectifier. The rectifier 20includes a positive lead 22 electrically connected to the anode 16 and anegative lead 24 electrically connected to the steel 18 (functionalcathode) to provide consistent/constant current density of substantially8-10 and more preferably 9 amps/dm². Preferably, electric current ispassed through the plating solution at a level of substantially 50 ampsat substantially 8 volts. This causes copper ions in the platingsolution 14 to form onto and plate the steel 18.

As the copper ions are consumed from the plating solution 14, they mustbe replaced. Since the anode 16 is insoluble, another source of coppermust be provided. To this end, a copper source is provided directly tothe plating solution 14. Specifically, the copper source is preferablysoluble in the plating solution 14. As the pyrophosphate platingsolution includes hydrogen ions (H⁺), conventional copper sources suchas copper (Cu), or copper oxide (CuO) are insoluble and will notfunction properly.

While copper carbonate (CuCO₃) is soluble in the pyrophosphate solution,it does not function in a desirable manner as its use leads to the rapidformation of carbonic acid. As a result, the change in pH occurring inthe plating solution 14 generated by plating cannot be neutralized bythe copper carbonate. Thus, the pH continues to be decreased eventuallyto a level effectively blocking the plating and corroding the steel 18.While chemicals may be added to the plating solution 14 to neutralizethe pH drop, carbonates still build up. Eventually, they could reachsufficiently high levels to precipitate and require cleaning from theplating tray. The downtime necessary to do this would adversely effectproductivity and, accordingly, copper carbonate is also not a suitablecopper source.

Copper hydroxide (Cu(OH)₂), however, may be utilized as the coppersource in the pyrophosphate plating solution 14. More particularly, whenthe pH of the pyrophosphate solution is maintained between 7-10, morepreferably between 8-9 and most preferably between 8.1-8.4, the copperhydroxide is soluble in the plating solution.

During plating, the pH of the solution 14 decreases. Specifically, asshown in the following formula, hydrogen ions are produced by theplating process:

    Cu.sup.2+ +H.sub.2 O⃡2H.sup.+ +1/2O.sub.2 ↑+Cu.sup.o (plated)

Advantageously, as shown in the following formulae, copper hydroxideprovides the necessary hydroxide ions to counteract or balance thehydrogen ions generated during plating:

    Cu(OH).sub.2 ⃡Cu.sup.2+ +2(OH).sup.-

leading to the neutralization reaction:

    2H.sup.+ +2OH.sup.- →2H.sub.2 O

Thus, when copper hydroxide is utilized as the source of copper in thepyrophosphate plating solution 14, the net reaction is:

    Cu(OH).sub.2 +H.sub.2 O⃡2H.sub.2 O+1/2O.sub.2 ↑+Cu.sup.o (plated).

Accordingly, it should be appreciated that as long as copper hydroxideis added to the plating solution 14 in sufficient quantities to replaceor offset the copper consumed from the solution in the plating process,the pH of this solution is maintained at a constant level.

In order to achieve this end, copper ions from the copper source aremaintained in the solution 14 at a concentration of between 14-22 gramsper liter and preferably substantially 20 grams per liter. Thus, wherecopper hydroxide is utilized as the copper source, a concentration ofbetween 21.5-33.8 grams per liter and preferably substantially 30.7grams per liter is maintained.

Of course, there will be some fluctuation in pH in the plating solution14 during plating due to momentary fluctuations in copper hydroxideconcentration, water evaporation and solution spillage. Accordingly, apH monitor 26, of any appropriate type known in the art, is provided tomonitor the pH of the solution in the plating tray 12. As the leveldecreases to the lower end of the desired range (pH 7-10), more copperhydroxide is added to the solution to maintain the necessary operatingparameters for continuing the plating process.

Preferably, the copper hydroxide is added through a conduit 28, leadingfrom a dissolving tank 30, by means of a pump 32. More specifically,copper hydroxide powder 34 is added into a copper pyrophosphate solution36 maintained at 45°-55° C. (preferably 50° C.) and having a pH between7-10 substantially corresponding to that in the plating tray 12. Anagitator (not shown) may be provided, if desired, to aid in thedissolving process. Additionally, a second pH monitor 38 is provided tomonitor the pH of the pyrophosphate solution 36 in the dissolving tank30. Thus, it should be appreciated that it is possible to add copperhydroxide in solution 36 at the desired concentration and pH from thedissolving tank 30 into the plating tray 12 as required to maintain thebest conditions for providing the highest quality plating.

Further, as an insoluble anode 16 is utilized that maintains its shapeat all times during the plating process, there are no significantfluctuations in current density as often occur when utilizing solubleanodes. Thus, consistent, even plating of uniform thickness is ensured.Additionally, the need to replace exhausted soluble anodes iseliminated. Accordingly, the potential for sparking and the platingdefects resulting therefrom are avoided.

Briefly summarizing, the method for plating steel with copper includesproviding a pyrophosphate plating solution 14 of the type described in aplating tray 12 during plating, the pH of the solution is maintainedbetween 7-10, more preferably between 8-9 and most preferably between8.1-8.4. This is accomplished by adding copper hydroxide to thesolution. The copper hydroxide advantageously provides both copper tothe solution for plating and hydroxide ions for reacting with hydrogenions produced during plating. In fact, the ratio of the presentation ofhydroxide ions to hydrogen ions is 1:1 so that a pH stable solution iseffectively provided. Finally, the plating process is driven by passingan electric current of substantially 50 amps at 8 volts through theplating solution. This provides a constant current density of 8-10 andpreferably 9 amps/dm². A high quality copper plated steel product P withuniform plate thickness is produced by this method.

An example is presented below to further illustrate the invention.

EXAMPLE 1

An insoluble metal anode (titanium anode coated with IrO₂, thickness 20g/m², as available from Nisshin Kasei Company of Japan, model NY type)was installed in an non-corrosive plating tray. Next, 250 l of platingsolution including 300 grams per liter of K₂ P₂ O₇, 56 grams per literCu₂ P₂ O₇, 20 grams per liter Cu(OH)₂ and 1 gram per literpolyphosphoric acid (total pyrophosphates 175 grams per liter), wasadded to the plating tray. The tray was 2,000 mm in length, 300 mm inwidth and 150 mm in depth. The solution was brought to and maintained ata temperature of 50° C. during the plating operation. The pH of thesolution was 8.1 at the start of the process and maintained as close tothat level as possible throughout. To achieve this end, the pH of thesolution was monitored utilizing a digital pH meter as available fromHoriba Seisakusho of Japan. As the pH began to drop during plating,copper hydroxide in a pyrophosphate solution was pumped from adissolving tank into the plating solution in the plating tray. Thedissolving tank had a capacity of 880 l and including a Cu content ofpowder of 61% allowing Cu deposit by plating of 100 kg/day. The amountof copper hydroxide utilized at this rate was 164 kg/day or 6.8 kg/hour.Maximum dissolving of copper was 5.0 grams per liter.

Steel wire, having a diameter of 1.68 mm was positioned in the platingsolution adjacent the top of the plating tray. The wire was movedthrough the plating tray at 2.6 m/min and had a dipping time of 45seconds. The wire and anode were connected to the negative and positiveleads, respectively of a rectifier (model FBS-080-500, available fromChuo Seisakusho of Japan) set to an amperage of 9.5 amps atsubstantially 5.0 volts. Accordingly, a constant current density of 9amps/dm² was provided. This produced a plating weight of 2.1 grams ofcopper per kilogram of steel. The resulting plating was consistentlyapplied and of uniform depth.

In summary, numerous benefits result from employing the concepts of thepresent invention. The method and apparatus simplify the plating processby eliminating the need to monitor and replace exhausted soluble copperelectrodes. Accordingly, the potential problem of sparking is avoided.Further, as an insoluble anode is utilized, a constant current densityresults. This means that the plating is uniformly and evenly appliedover the steel being processed.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Obvious modifications or variations are possible in light ofthe above teachings. The embodiment was chosen and described to providethe best illustration of the principles of the invention and itspractical application to thereby enable one of ordinary skill in the artto utilize the invention in various embodiments and with variousmodifications as is suited to the particular use contemplated. All suchmodifications and variations are within the scope of the invention asdetermined by the appended claims when interpreted in accordance withbreadth to which they are fairly, legally and equitably entitled.

We claim:
 1. A method for electrolytically plating copper onto a basemetal utilizing a plating apparatus including a power source and aplating tray, comprising the steps of:providing a pyrophosphate platingsolution in said plating tray; providing an insoluble anode in saidplating solution; passing an electric current through said platingsolution between said insoluble anode and said base metal; and addingcopper hydroxide to said plating solution to maintain a concentration ofcopper hydroxide in said plating solution of at least 21.5 grams/literand a pH between substantially 7-10 so that a substantially constantcurrent density is maintained and copper from said solution is platedonto said base metal in a uniform manner.
 2. The method set forth inclaim 1, wherein said pyrophosphate plating solution is formed fromwater, 200-400 grams per liter of tetrapotassium pyrophosphate (K₄ P₂O₇) and 40-60 grams per liter of copper pyrophosphate (Cu₂ P₂ O₇). 3.The method set forth in claim 2, wherein said pyrophosphate platingsolution further includes between 0-3 grams per liter of polyphosphoricacid.
 4. The method set forth in claim 1, including maintaining aconcentration of 21.5-33.8 grams per liter of copper hydroxide in saidplating solution.
 5. The method set forth in claim 4, includingmaintaining said plating solution at a temperature of between 45°-55°.6. The method set forth in claim 4, including passing electric currentthrough said plating solution between said insoluble anode and said basemetal to be plated at substantially 50 amps at substantially 8 volts toprovide a current density of substantially 8-10 amps/dm².
 7. The methodset forth in claim 1, including maintaining a concentration of copper insaid plating solution between 14-22 grams per liter.
 8. The method setforth in claim 1, including maintaining a concentration of copper insaid plating solution at substantially 20 grams per liter.
 9. The methodset forth in claim 1, including maintaining a concentration of 30.7grams per liter of copper hydroxide in said plating solution.
 10. Themethod set forth in claim 9, including maintaining the temperature ofsaid plating solution between 45°-55° C.
 11. The method set forth inclaim 10, including passing electric current through said platingsolution between said insoluble anode and said base metal to be platedat substantially 50 amps at substantially 8 volts to provide a currentdensity of substantially 8-10 amps/dm².
 12. A copper plated productproduced in accordance with the method set forth in claim
 1. 13. Acopper plating solution, comprising:200-400 grams per liter oftetrapotassium pyrophosphate; 40-60 grams per liter of copperpyrophosphate; 21.5-33.8 grams per liter of copper hydroxide; and 0-3.0grams per liter of polyphosphoric acid; said plating solution alsohaving a pH between 7-10.
 14. A copper plating system for copper platinga base metal, comprising:(a) a bath including;a pyrophosphate platingsolution providing a source of copper for plating, said source of copperbeing copper hydroxide at a concentration of between substantially21.5-33.8 grams/liter and said plating solution having a pH betweensubstantially 7-10; and (b) an apparatus including;a plating tray forholding the pyrophosphate plating solution; an insoluble anode receivedin the pyrophosphate plating solution held in the plating tray; and apower source for passing electric current through said pyrophosphateplating solution between said anode and said base metal to plate copperfrom said solution onto said base metal in a uniform manner.
 15. Thecopper plating system set forth in claim 14, wherein said pyrophosphateplating solution is formed from tetrapotassium pyrophosphate and copperpyrophosphate.
 16. The copper plating system set forth in claim 15,wherein said pyrophosphate plating solution includes between 200-400grams per liter of tetrapotassium pyrophosphate and 40-60 grams perliter copper pyrophosphate.
 17. The copper plating system set forth inclaim 15, wherein said pyrophosphate solution includes substantially 300grams per liter tetrapotassium pyrophosphate and substantially 56 gramsper liter copper pyrophosphate.
 18. The copper plating system set forthin claim 14, wherein said power source is a rectifier providingsubstantially 50 amps at substantially 8 volts.
 19. The copper platingsystem set forth in claim 14, further including a dissolving tank fordissolving copper hydroxide into said plating solution.
 20. The copperplating system set forth in claim 19, further including a conduitproviding fluid communication between said dissolving tank and saidplating tray and a pump for supplying plating solution from saiddissolving tank to said plating tray.
 21. The copper plating system setforth in claim 14, further including a pH monitor for monitoring said pHof said plating solution in said plating tray.